As the information age advances, display devices for displaying information are actively being developed. More particularly, flat panel display (FPD) devices having a thin profile, light weight and low power consumption are actively being pursued to substitute cathode ray tube (CRT) devices. For example, a liquid crystal display (LCD) device, a plasma display panel (PDP), a field emission display (FED) device and an electroluminescent display (ELD) device have been researched and developed as an FPD device. Specifically, liquid crystal display (LCD) devices are widely used as monitors for notebook computers and desktop computers because of their high resolution, high contrast ratio, color rendering capability and superiority in displaying moving images.
A liquid crystal display (LCD) device relies on optical anisotropy and polarizability of liquid crystal molecules to produce an image. Due to the optical anisotropy of liquid crystal molecules, refraction of light incident onto the liquid crystal molecules depends on the alignment direction of the liquid crystal molecules. Liquid crystal molecules are aligned with directional characteristics resulting from their long, thin shapes. The alignment direction of the liquid crystal molecules can be controlled by applying an electric field thereto. An LCD panel includes two substrates and a liquid crystal layer interposed therebetween. The liquid crystal molecules are aligned according to the direction of an electric field generated between electrodes disposed on both substrates of the LC panel. By refracting and transmitting incident light and controlling the electric field applied to a group of liquid crystal molecules within particular pixel regions, a desired image can be obtained. However, because an LCD panel does not emit light, an LC module or an LCD device requires an additional light source. Accordingly, an LCD module includes a backlight unit disposed below an LCD panel to supply light.
In general, backlight units may be classified into two types according to a disposition of the light source, such as a side-type and a direct-type. In a side-type backlight unit, a light guide plate (LGP) is disposed at a rear of an LCD panel and a light source is disposed at a side of the LGP. Light emitted from the light source is refracted in the LGP and is supplied to the LCD panel. In a direct-type backlight unit, a plurality of light sources are disposed at a rear of an LCD panel, and light emitted from the plurality of light source is directly supplied to the LCD panel.
A cold cathode fluorescent lamp (CCFL) and an external electrode fluorescent lamp (EEFL) have been used as a light source of a backlight unit. Recently, a light emitting diode (LED) is suggested as a light source of a backlight unit because of its high color reproducibility and high brightness. A backlight unit using an LED as a light source may be referred to as an LED backlight unit.
FIG. 1 is a cross-sectional view showing a liquid crystal display module including a side-type light emitting diode backlight unit according to the related art. In FIG. 1, an LCD module includes an LCD panel 10, an LED backlight unit 20, a bottom frame 50, a main frame 40 and a top frame 60. The LCD panel 10 and the LED backlight unit 20 are modularized by a mechanism to prevent damage by an external impact and to minimize light loss. The main frame 40 surrounds the LCD panel 10 and the LED backlight unit 20. The bottom frame 50 surrounds the main frame 40 and covers a rear surface of the LED backlight unit 20. The top frame 60 surrounds a front edge of the LCD panel 10. The top frame 60 and the bottom frame 50 are combined through the main frame 40.
The LCD panel 10 includes first and second substrates 2 and 4, and a liquid crystal layer (not shown) is interposed between the first and second substrates 2 and 4. The LED backlight unit 20 includes a reflecting sheet 22, a light guide plate 24, a plurality of optic sheets 26 and a plurality of light emitting diodes (LEDs) 30. The reflecting sheet 22 is disposed on an inner surface of the bottom frame 50, and the light guide plate 24 is disposed on the reflecting sheet 22. The plurality of optic sheets 26 are disposed between the light guide plate 24 and the LCD panel 10. The plurality of LEDs 30 formed on a printed circuit board (PCB) 32 is disposed along a side surface 25 of the light guide plate 24.
Light emitted from each LED 30 passes through the side surface 25 of the light guide plate 24 and is refracted toward the LCD panel 10 in the light guide plate 24. While the refracted light and reflected light on the reflecting sheet 22 pass through the plurality of optic sheets 26, the light is treated to become plane light having high uniformity and high quality and is supplied to the LCD panel 10.
FIG. 2 is a magnified view of a portion “A” of FIG. 1. In FIG. 2, the plurality of LEDs 30 are disposed on the side surface 25 of the light guide plate 24. The plurality of LEDs 30 include red, green and blue (RGB) LEDs that are periodically arranged. The RGB LEDs are turned on together to generate white-colored light. To reduce a power consumption and circuit area, the plurality of LEDs 30 are formed on the PCB 32 as an array including several LEDs. The PCB 32 is fixed using an adhesive so that the plurality of LEDs 30 can face the side surface 25 of the light guide plate 24. Accordingly, the main frame 40 has a vertical surface 42 facing the side surface 25 of the light guide plate 24 and the PCB 32 is fixed to the vertical surface 42 of the main frame 40 using an adhesive 62 such as a double stick tape.
In addition, the LCD module includes a backlight driving circuit 70 (of FIG. 1) for driving and controlling the plurality of LEDs 30, and the backlight driving circuit 70 (of FIG. 1) is disposed on a rear surface of the bottom frame 50 (of FIG. 1) to minimize a volume of the LCD module. Accordingly, an additional connecting line 72 is required to electrically connect the PCB 32 having the plurality of LEDs 30 and the backlight driving circuit 70. The connecting line 72 extends from the PCB 32 through the main frame 40 or the bottom frame 50 to exterior to be connected to the backlight driving circuit 70.
An internal temperature of an LED abruptly increases according to a turn-on time, and the increase in temperature causes a change in brightness of the LED. FIG. 3 is a graph showing a relationship between internal temperature and brightness in an LED according to the related art. In FIG. 3, as the internal temperature of the LED increases, the brightness of the LED decreases with a different slope according to a color of the LED. For example, when the green LED has the internal temperature over 80° C., the brightness of the green LED is less than 80% of the brightness of the green LED having the internal temperature of 25° C. In addition, when the internal temperature of the green LED is over 120° C., the green LED does not emit light any more.
Accordingly, the design for heat radiation is a factor in use of an LED as a light source of a backlight unit. Specifically when a plurality of LEDs are disposed on a PCB, the increase in the internal temperature and the decrease in the brightness become severer. However, an LCD module according to the related art does not have an effective means for radiating heat generated in an LED, and the decrease in the brightness deteriorates a display quality. The adhesive 62 (of FIG. 2) for fixing the PCB 32 (of FIG. 2) may be formed of a heat-transmissive material for heat radiation. However, the effect of the heat-transmissive adhesive 62 (of FIG. 2) is not satisfactory. Moreover, due to the heat-transmissive adhesive 62 (of FIG. 2), the material cost increases and the fabrication process is complicated.
Referring again to FIG. 2, since the connecting line 72 is exposed outside the bottom frame 50 for connecting the PCB 32 inside the LCD module and the backlight driving circuit 70 outside the LCD module, the connecting line 72 may be cut or separated from the LCD module while the LCD module is transported or used. In addition, since a through hole is formed in the bottom frame 50 and the main frame 40 for extending the connecting line 72 from inside to outside of the LCD module, an additional forming step for the through hole is required in fabrication process.